1. Field of Invention
This invention is in the field of methods and apparatus for semiconductor processing and, more particularly, relates to a method and apparatus for conditioning a pad used for polishing a silicon wafer.
2. Description of Prior Art
A silicon wafer is typically fabricated as a disc having a diameter in a range of 100 to 200 millimeters and a thickness in a range of 16 to 20 mils. The wafer is thereafter subjected to a masking process in preparation for using it, for example, in a production of integrated circuits.
The masking process causes a multiplicity of undesired irregularities on a device surface of the wafer. It should be appreciated that the size of the irregularities is typically on the order of one micron. However, it is of critical importance that the irregularities be removed.
The removal of the irregularities is accomplished through the use of a polishing pad having a circular polishing surface. The diameter of the surface of the polishing pad is up to several times as large as the diameter of the wafer.
The device surface is made to bear against the polishing pad surface near the edge thereof. The wafer and the pad both axially rotate, causing the polishing pad surface to rub against the device surface and thereby polish it. The polishing is enhanced by a process that includes dispersing a slurry, typically comprised of a colloidal silica, on the polishing pad surface. This enhanced polishing process, known as chemical mechanical planerization (CMP), is usually effective in removing the irregularities.
The slurry and the removed irregularities undesirably impregnate the pad, causing the polishing pad surface to become smooth and irregular, in a manner similar to an automobile polishing cloth becoming smooth and irregular after extensive use. The impregnated pad may be conditioned for polishing by using what is known as an end effector which rotates as it bears against the polishing pad surface.
The end effector is usually connected to one end of a cantilever that is rotatable about the axis of a mounting arrangement adjacent to the pad. The mounting arrangement comprises the other end of the cantilever. The end effector moves in an arcuate path in contact with the polishing pad in a manner similar to a phonograph needle moving over a phonograph record mounted on the turntable of a record player.
A disadvantage of the arcuate path is that when the polishing pad is axially rotated, locations on the polishing pad surface have linear velocities that are directly related to their respective radii. Hence, the respective linear velocities of the locations are non-uniform. Therefore, the conditioning is non-uniform, thereby causing the polishing to be correspondingly non-uniform.
Heretofore, simple and inexpensive apparatus and a method for uniformly conditioning the polishing surface have been unknown.